Split presubmit_support.Main() in two to make it simpler.

Handle better the case with unversioned files and add a unit test.

R=dpranke@chromium.org

Review URL: http://codereview.chromium.org/6674059

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@78700 0039d316-1c4b-4281-b951-d872f2087c98